This invention relates to an anti-crevice corrosion sealant for use in sealing parts in sea water conversion plants, pumps, refrigerators, heat exchangers, and their related pipings and also to a method for anti-crevice corrosion.
In pipings for transferring an aqueous medium such as sea water and industrial water, their joint parts are sealed with rubbery sheets or asbesto sheets or their rings, but there still remain fine crevices at the joint parts. At bolt-secured parts and jointed parts between one plane member and another in the ordinary machines and apparatuses, there remain likewise fine crevices.
In the fine crevices, mass transfer is difficult to occur, and a definite amount of the dissolved oxygen contained in the aqueous medium in the crevices is consumed for forming a passive state film on metal surfaces, but its supplementation is difficult. Thus, on the metal surfaces in the fine crevices, which will be hereinafter referred to as "crevice inner surfaces", we have such a fear that some of metal surfaces are exposed to the aqueous medium without coating with a passive state film due to unsatisfactory supplementation of dissolved oxygen. On the other hand, on the outer surfaces adjacent to the crevice inner surfaces complete passive state films are formed due to satisfactory supplementation of dissolved oxygen.
Under these circumstances the crevice inner surfaces take an electrochemically less noble electrode potential to the aqueous medium, whereas the outer surfaces adjacent to the crevice inner surfaces take an electrochemically noble electrode potential to the aqueous medium, and thus a potential difference arises between the crevice inner surfaces and the adjacent outer surfaces to activate and dissolve the crevice inner surfaces. Thus, on the adjacent outer surfaces reduction reaction (cathode reaction) of dissolved oxygen takes place correspondingly to the electric charges on the crevice inner surfaces. Furthermore, in the crevices, a hydrogen ion concentration is increased by the dissolution of crevice inner surfaces, and the aqueous medium in the crevices turns acidic. Under these situations, the passive state films on the crevice inner surfaces become unstable, and the films are dissolved, with the result that crevice corrosion is accelerated.
Generation of the crevice corrosion is due to the fact that the crevice inner surfaces are anodically polarized, whereas the adjacent outer surfaces are cathodically polarized to dissolve the crevice inner surfaces and the aqueous medium in the crevices turns acidic.